Wireless communications systems, including cellular phones, paging devices, personal communication services (PCS) systems, and wireless data networks, have become ubiquitous in society. Wireless service providers continually try to create new markets for wireless devices and to expand existing markets by making wireless devices and services cheaper and more reliable. The price of end-user wireless devices, such as cell phones, pagers, PCS systems, and wireless modems, has been driven down to the point where these devices are affordable to nearly everyone.
To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base station (BS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof.
Wireless equipment manufacturers currently are developing wireless local area networks (WLANs) that are intended to deliver high-speed data to wireless devices, including cell phones and wireless laptops. However, consistently achieving high throughputs to wireless devices has proved elusive in many respects due to interference, multipath fading, and other effects. To overcome these problems, improvements are being made in many areas, including, medium access control (MAC) techniques, radio frequency (RF) components, modulation techniques, and smart antenna systems.
Smart antenna systems that use multiple antennas for both transmitting and receiving are particularly effective in increasing the data rate between a wireless network and a wireless terminal (or mobile station). Such multiple antenna systems are often called multiple-input, multiple-output (MIMO) antenna systems. A MIMO system has numerous advantages over a conventional single-input, single-output (SISO) antenna system, particularly in radio frequency (RF) environments that exhibit a large amount of multipath.
In a MIMO antenna system, a high-rate data stream is divided into multiple lower-rate data streams. The lower-rate data streams are individually modulated and transmitted through separate antennas at the same time and using the same frequency channel. The multiple transmitted signals experience multipath reflections as the transmitted signals travel from the multiple antennas of the base station to the multiple antennas of the receiving mobile station. The output of each receiver antenna is a linear combination of the multiple transmitted RF signals. Within the receiver, the multiple lower-rate data streams are detected and recovered by well-known MIMO algorithms.
Using multiple antenna arrays takes advantage of the spatial dimension of RF channel in order to increase throughput. This is known as spatial multiplexing and it provides a multifold increase in throughput. For example, using an N antenna array at the base station to transmit to an N antenna array at the mobile station increases the peak throughput of a MIMO antenna system by a factor of N over a conventional SISO antenna system. Advantageously, the MIMO antenna system has the same frequency bandwidth as the SISO antenna system. As a result, a MIMO antenna system provides a higher data rate to each mobile station and decreases delays in the RF link.
However, the primary drawback to using a MIMO antenna system is the physical size of the antenna array. In order to gain the benefit of a MIMO antenna system, it is preferable that the antennas have a significant spatial separation—on the order of ten (10) wavelengths. At the frequencies of interest, this requires an antenna separation on the order of a meter. This is highly impractical for nearly all cell phone and laptop computer implementations. In these cases, the antenna would be several times the size of the mobile phone or mobile PC.
Therefore, there is a need in the art for improved wireless networks having increased data throughput. In particular, there is a need for a wireless network that uses multiple-input, multiple-output (MIMO) antenna arrays to transmit data to a cell phone or other small-sized mobile station without requiring an oversized MIMO antenna at the cell phone or other mobile station.